progress bar for IE10+/Edge (border radius, use ::-ms-fill, variant mixins, striped)

clarify lack of support for striped and animated striped bars in
IE10+/Edge

Remove fully supported features from IE9 limitations docs

If these are supported, why bother mentioning them in a section that's about communicating what's not supported?
These rows made sense when the section also covered IE8 (http://getbootstrap.com/getting-started/#support-ie8-ie9 ),
which doesn't support these features, but we no longer care about IE8 in v4.

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Docs: Include major version in `npm install bootstrap` commands
Future-proofing for when Bootstrap v5 eventually exists.
Refs #18520

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Add Wall of Browser Bugs entry for #18504

https://connect.microsoft.com/IE/feedback/details/2110930/edge-ie11-gradient-background-image-on-tr-is-only-applied-to-first-td

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Fixes #18505
Closes #18509

Remove unnecessary style attribyte on docs (aligning-images)

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Updated CDN URLs in v4-dev docs

Errata from:
* 8ce5da30
* 5d3144e8
which missed a few numbers.

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Remove !default Scss flag in docs grid example

Fix utility page code example Markdown

v4 alpha 2

/fyi @mdo There might be prettier/better ways of explaining these,
        but for the sake of having accurate docs for tomorrow,
        this should do in a pinch.

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Refactor _utilities-spacing.scss

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